Question

If a diffraction grating has 3700 lines per cm, what is the spacing d between lines

Answer 1

so first we find the gap between the slits by the formula d=1/N 

N is number of lines per metre so 3700 line/cm = 370000 lines/m 
So d=2.7*10^-6 

Now we use the formula dsin(angle)=n(wavelength) 

d is the same 
n is the order of the diffraction pattern 

so wavelenth=dsin(angle)/n 
=[(2.7*10^-6)*sin30]/3 
=4.5*10^-7 m